1. Field of the invention
This invention relates to a packaging material for photosensitive materials for photographic purpose.
2. Description of prior arts
Various types of packaging materials for photo sensitive materials for photographic purposes (hereafter simply referred to as "photosensitive materials") have widely been put to practical use, and various properties are required according to their uses.
As the packaging materials for photosensitive materials, it is necessary various properties such as light-shielding, gas barrier, moisture proofing, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, hot tack properties (hot-seal ability), and seal ability of contraries, bag-making aptitude, ply peeling resistance, slipping character and the like. Generally, it is difficult to satisfy these properties by a single material, and a single layer film of a high-pressure low-density polyethylene (LDPE) kneaded with carbon black or a pigment. Therefore, composite laminated film composed of LDPE film containing carbon black or a pigment dispersed therein, and flexible sheet such as paper aluminum foil or cellophane, etc. has been employed. An example of the conventional laminated film is shown in FIG. 11. This film is composed of a light-shielding LDPE film layer 11a, a light-shielding metal foil layer 8a laminated on it through an adhesive layer 2, and a flexible sheet layer 6 laminated thereupon through an adhesive layer 2. Another example of the conventional film is shown in FIG. 12. This film was used for packaging color photographic printing paper, and it is composed of a light-shielding LDPE film layer 11a, a light-shielding metal foil layer 8a, a flexible sheet layer 6, and a light-shielding LDPE film layer 11a. They are laminated in that order, and an adhesive layer 2 is provided between each layers.
On the other hand, the present inventor has already disclosed a laminated film for photosensitive materials of which physical strength was raised by combining two uniaxially stretched films (U.S. Pat. No. 4,331,725). Another laminated film for photosensitive materials shown in FIG. 13 has also been disclosed. This film is composed of a foamed sheet layer 12 and two uniaxially oriented thermoplastic resin film layers 13a, 13a having a light-shielding property and laminated on both faces of the foamed sheet layer 12 directly or through an adhesive layer 2. Thickness of the foamed sheet layer 12 is in the range of 0.3 to 2.0 mm, and expansion ratio is 5 to 50 times. Two uniaxially oriented films 13a, 13a are located so that their molecular orientation axes cross each other at an angle of more than 30 degrees, and the laminated film is compressed up to 40 to 85% of theoretical total thickness. Impact puncture strength and Gelbo test strength of the compressed laminated film is large, and curling hardly occurs. This film is suitable for a heavy material.
Though the above laminated films were made in order to improve the foregoing physical properties, physical properties of these conventional films were still not sufficient and during packaging, the films were sometimes torn or punctured or during heat sealing of the films they sometimes separated, particularly in the cases of heavy products and roll films having a sharp edge. In addition, when a large amount of a light shielding material such as carbon black was added, physical strength of the film was lowered. Then, the amount of carbon black was set at about 3%, and total thickness of the film was more than 70 .mu.m. As the result, the film was stiff, and working efficiency of the packaging process was reduced. Cost of the packaging was also expensive. In the case of the laminated film shown in FIG. 12, tear strength, impact puncture strength, Gelbo test strength, etc. were not enough. In the case of the laminated film disclosed in the specification of U.S. Pat. No. 4,331,725, since two layers of uniaxially stretched film were necessary, the laminated film was thick and expensive as a packaging material, though physical properties such as tear strength were improved. In the case of the laminated film shown in FIG. 13, the laminated film was thick and its heat sealing properties were not good in order to secure light-shielding and moisture proofing. Furthermore, since bag-making properties were also not good bags for packaging photosensitive materials was formed by using a heat sealer having a special cooling device, and a hot bar was pressed through a heat-resistant film. As a result of these, the laminated film of FIG. 13 was expensive.